The invention relates to the production of aluminum by igneous electrolysis according to the Hall-Hxc3xa9roult process, and more particularly to methods and means of implementing it industrially. In particular, the invention relates to lines of electrolysis pots laid out crosswise, in other words with their long sides perpendicular to the centerline of the line.
1. State of the Art
Metal aluminum is produced industrially by igneous electrolysis, namely by the electrolysis of aluminum in solution in a molten cryolith bath called an electrolysis bath according to the well known Hall-Hxc3xa9roult process. The electrolysis bath is contained in a pot including a steel shell lined on the inside with refractory and/or insulating materials, and a cathodic assembly located at the bottom of the pot. Anodes made of carbonaceous material are partially immersed in the electrolysis bath. The pot and the anodes form what is frequently called an electrolysis cell. The electrolysis current that passes through the electrolysis bath and the liquid aluminum layer through the anodes and cathodic elements, brings about alumina reduction reactions and also keeps the electrolysis bath at a temperature of the order of 950xc2x0 C. by the Joule effect.
In order to maintain the profitability of a plant, efforts are made firstly to reduce investments and operating costs, and secondly to obtain the highest possible current intensities and current efficiencies at the same time, while protecting and even improving operating conditions of the electrolysis cells.
Consequently, the most modern plants contain a large number of electrolysis cells laid out in line in xe2x80x9celectrolysisxe2x80x9d pot rooms electrically connected in series using connecting conductors in order to optimize the occupancy of factory floors. The pots, that are almost always rectangular in shape, are usually laid out side by side, in other words with the long sides perpendicular to the center line of the line (it is also said that they are laid out xe2x80x9ccrosswisexe2x80x9d) but they may also be placed head to head (in this case they are said to be laid out xe2x80x9clengthwisexe2x80x9d). The pots are usually arranged to form two or several parallel lines that are electrically connected to each other by end conductors. The electrolysis current thus passes in cascade from one cell to the next. The length and weight of the conductors are as small as possible in order to limit the corresponding investment and operating costs, particularly through a reduction of Joule effect losses in conductors. Furthermore, bringing electrolysis pots closer together and increasing the intensities of electrolysis current has led to the development of conductor configurations capable of compensating for the effects of magnetic fields generated by the electrolysis current.
With the same objective, it is known that pots, or lines of pots, can be provided with sophisticated regulation means that enable good control of the electrolysis process. In particular, French application FR 2 753 727 filed by the applicant proposes a detailed temperature regulation process that can give high values of the current efficiency.
Electrolysis pots are usually controlled such that they are in thermal equilibrium, in other words the heat dissipated by each electrolysis pot is globally compensated by the heat produced in the pot, which originates essentially from the electrolysis current. Thermal equilibrium conditions depend on the physical parameters of the pot such as the dimensions and the nature of the materials from which the pot is made, and the pot operating conditions, such as the electrical resistance of the pot, the bath temperature or the intensity of the electrolysis current. The pot is frequently made and operated so that a ridge of solid bath is formed on the sidewalls of this pot, which in particular inhibits attack of the linings of the said walls by the liquid cryolith. The thermal equilibrium point is usually chosen such that the best operating conditions are achieved both technically and economically.
French patent FR 2 552 782 (corresponding to American patent U.S. Pat. No. 4,592,821) in the name of the applicant describes a line of electrolysis pots that can operate industrially at current intensities exceeding 300 kA and with current efficiencies exceeding 90%.
2. Statement of the Problem
The continuous improvement in the performances of electrolysis plants, both technically and economically, has led the applicant to search for global solutions for increasing the cost effectiveness of plants, particularly by allowing for the possibility of a range of pot operating intensities. The possibility of making deliberate variations to operating conditions, which may be quite different from nominal conditions, is often useful in the management of an electrolysis plant. For example, an attempt can be made to vary the power of the series of electrolysis pots as a function of an electrical energy contract.
The applicant has found that electrolysis pots have temperature heterogeneities and more precisely a dispersion of temperature values within the liquid mass which, although relatively small, tend to be constant over time, in other words some differences between local temperatures and the average temperature of the pot are not cancelled by averaging over in time. In particular, these heterogeneities have the disadvantage that they limit the accuracy of the temperature regulation of the pots. Known regulation processes can control temperature fluctuations in time, but do not necessarily limit the dispersion of temperatures over the entire pot. Furthermore, zones in which the temperature is below the said temperature encourage material deposits at the bottom of the pot and the formation of extending ridges (in other words part of the ridge partially covers the cathode) that increase the cathodic voltage drop and are the cause of pot instabilities, and zones in which the temperature is higher than the set temperature, tend to reduce the protective solidified bath ridges on the sides of the pot and possibly lead to non-uniform wear of the linings.
Therefore, the applicant searched for means of reducing the temperature dispersion and temperature fluctuations in electrolysis pots that would overcome the disadvantages of prior art while remaining satisfactory for the general pot design, particularly concerning floor occupancy and investment in operating costs, and for operation of the pots.
3. Purpose of the Invention
The first object of the invention is an arrangement of electrolysis pots laid out crosswise for the production of aluminum by igneous electrolysis according to the Hall-Hxc3xa9roult process.
Another object of the invention is an electrolysis plant including an arrangement of pots according to the first object of the invention.
According to the invention, the arrangement of electrolysis pots for the production of aluminum by igneous electrolysis according to the Hall-Hxc3xa9roult process with an electrolysis current with intensity To includes at least one first line of electrolysis pots forming a first electrical circuit and at least one second electrical circuit located at a determined average distance from the said first line, the said first line including N pots arranged crosswise and connecting conductors to transmit the said electrolysis current Io from a pot in the first line called the upstream pot, to the next pot in the said line called the downstream pot, each pot including a metal shell, internal lining elements, anodes and cathodic elements, the said cathodic elements being provided with cathodic connection ends projecting on the upstream side and the downstream side of the shell of each pot, a first part Im of the current Io being output through the cathodic ends projecting from the upstream side of each pot, a second part Iv of the current Io being output through the cathodic ends projecting from the downstream side of each pot, the said connecting conductors including rising conductors called xe2x80x9crisersxe2x80x9d, the current Io output from all cathodic elements in an upstream pot being transmitted to the anodes of the downstream pot through the said risers, and characterized in that at least one xe2x80x9caxialxe2x80x9d conductor passes under each upstream pot in the central zone, in that at least one xe2x80x9clateralxe2x80x9d conductor passes under each upstream pot in the inner lateral zone, in other words the zone of each pot located on the side of the said second electrical circuit, in that at least one xe2x80x9cbypassxe2x80x9d conductor goes around each upstream pot, in that the or each lateral conductor is connected to a first set of the said cathodic ends located on the upstream side in order to transmit a first part I1 of the current Im, between 10 and 20% of the said current Im, to the said risers, and in that the or each axial conductor is connected to a second set of the said cathodic ends located on the upstream side in order to transmit a second part I2 of the said current Im, between 10 and 20% of the said current Im, to the said risers, in that the or each bypass conductor is connected to a third set of the said cathodic ends located on the upstream side in order to transmit a third part I3 of the current Im corresponding to the rest of the current Im, in that the said risers are connected to the cathodic ends located on the downstream side of the corresponding upstream pot, to the conductors passing under the said pot and to the or each bypass conductor of the said pot, such that a fraction Mc of the current Io less than 15%, and preferably less than 10%, is transmitted through the risers located in the central part of the line.
The lateral and central zones of the pot and of the line are delimited by two imaginary vertical planes parallel to the centerline of the line. Each of the said planes intersects the pots so as to form three zones corresponding to three comparable volumes of liquid mass inside each pot in the line. Preferably, the central volume is between 25 and 40% of the total volume, and preferably between 30 and 35% of the total volume. The exact volume of each zone, and the exact distribution of the current under the pot, depend on the pot structure (particularly the number of cathodic ends) and the pot operating mode (particularly the thickness of the solidified bath ridges on the edges of the pot crucible, which will modify the distribution of the liquid masses).
The said second electrical circuit also called the xe2x80x9cneighboring linexe2x80x9d in the rest of the text is usually approximately parallel to the line and usually includes at least one electrolysis pot. It usually includes a line of electrolysis pots, but it may possibly be composed of conductors only. During operation, a current with intensity Ioxe2x80x2 circulates in the said second circuit. The arrangement of the pots is preferably such that the intensities of the currents Io and Ioxe2x80x2 are approximately equal and that they are in directions opposite to each other.
The upstream current to the electrolysis pots is shared between the conductors depending on the intensity of the current in line Io and the intensity of the current in the neighboring line Ioxe2x80x2, and the distance between the two lines of pots.